The most common form of inhaler propels the dose of medicament in pressurised gas from an aerosol. However, this form of inhaler is becoming less popular because of environmental and other considerations. The delivery of some drugs in a dry, finely divided form has been shown to have certain medical advantages over other forms of delivery.
Some known proposals for delivering medicament in a finely divided form cannot be operated by one hand. It is considered that one-handed operation should be an important feature of such an inhaler.
EP 079478, EP 166294 and GB 2165159 all disclose dry powder inhalers which can be operated by one hand. In each of these proposals the inhaler includes a medicament storage chamber and an inhalation passage through which air is drawn via a mouthpiece. A metering member provided with a metering recess transfers a dose of medicament from the storage chamber and deposits it in the inhalation chamber. It is considered that the accuracy of such an arrangement can be very poor: on the one hand, by repeated indexing of the metering member it is possible to deposit two or more doses of medicament into the inhalation passage resulting in the administration of an overdose of medicament; on the other hand, since the medicament normally drops from the metering recess into the inhalation passage under gravity, particles of medicament can adhere to the interior of the metering recess so that an underdose is delivered.
U.S. Pat. No. 2,587,215 also discloses dry powder inhalers with the same disadvantages as those mentioned above. However, this document also discloses an embodiment in which the metering member presents the medicament in an upwardly open dispensing cup to a mixing chamber where it is mixed with air before being sucked into an inhalation tube via a nozzle having a narrow opening. Air sucked into the inhaler passes into the inhalation tube either directly or through the mixing chamber and nozzle. Accordingly, not all the air passes over the dispensing cup. If medicament adheres to the surface of the dispensing cup but is not sucked therefrom, there will be an underdose of medicament delivered to the user. It is considered that, upon repeated use of the hollows to deliver doses to the inhalation passage, a continually increasing amount of the powder will adhere to the base of the hollow, resulting in progressively decreased dosage to the patient. The tendency for a build-up of adherent powder to occur is thought to be a source of inaccurate dosing in many of the inhalers previously proposed. The metering member is a rotary sliding device journalled on a cylindrical pivot member extending from the bottom of the body of the device. Such an arrangement is susceptible to jamming due to ingress of powder between the cylindrical contacting surfaces of the pivot member and the metering member.
Another form of inhaler which is currently available includes a metering member including a number of tapered metering recesses which are open at top and bottom. In use, finely divided medicament from a storage chamber is packed into the recesses whereupon the metering member is moved to a dispensing position in which air can be drawn through the recesses to draw out the medicament. This device is considered to have a number of major shortcomings. Firstly, the metering recesses are prone to clogging. Secondly, a large amount of suction is required so that the device is unsuitable for many patients with breathing problems. Thirdly, two hands are required to operate the device.
The aim of the present invention may be viewed as being to provide a form of inhaler for one-handed operation which is capable of administering accurate doses and avoids the risk of multiple dosing. A further aim is to provide an inhaler which does not require a large amount of suction for effective operation.